Work is being continued on the synthesis of analogs of folic acid, pteroic acid, and their 4-amino analogs in which the nitrogen in the 10-position has been replaced by sulfur. 10-Thiaaminopterin and some compounds related to it turn out to have anti-bacterial activity and ability to inhibit leukemia cells in tissue culture comparable to that of aminopterin. Biological studies and studies of these analogs to interact with dihydrofolate reductase and with antibodies to methotrexate are being continued. The synthesis of analogs of aminopterin and 10-thiaaminopterin in which the sidechain is attached to the 7- rather than to the 6-position of the pteridine ring are in progress.